1. Field of the Invention
The present invention relates to a resonance-type semiconductor optical modulator with an asymmetrical electrode structure that can be used in optical communications as an intensity modulator that uses a small microwave modulating signal.
2. Description of the Prior Art
An optical modulator is a device that converts electrical signal information into optical information, such as intensity-, phase- or frequency-modulated information. Modulation methods include the direct method in which a source laser is directly modulated and the indirect method in which the source light wave is modulated by an external modulator. The direct modulation method can be realized with a simple system configuration, while the indirect method using an external modulator provides high-quality modulation. For communications involving ultrahigh speeds and long distances, modulation is usually carried out using an external modulator. Such external modulators used include those that utilize the electro-optical effect and those that utilize the semiconductor electro-absorption effect. The present invention utilizes the semiconductor electro-absorption effect, as described in further detail below.
A semiconductor optical modulator uses the Stark effect in a multi-quantum-well (MQW) structure, or the Franz-Keldysh effect that uses the dependence of the semiconductor fundamental absorption spectral on the electric field strength. As a consequence, the wavelength of the modulating light signal has to be substantially in accord with the fundamental absorption spectrum of the semiconductor. In some cases, the source laser is fabricated on the same semiconductor substrate.
A semiconductor optical modulator using the above effects comprises a p-type semiconductor, an n-type semiconductor and a semiconductor layer sandwiched between the p-type and n-type semiconductor layers. Focusing on the p-type and n-type semiconductor layers, in order to extend the absorption spectrum to the long-wavelength side, generally a bias voltage is applied to produce a reverse bias. The modulator uses the changed wavelength region to intensity-modulate the light wave.
FIG. 9 shows the configuration of a conventional semiconductor optical modulator To match the impedance of the feeding line for modulating signal and the impedance of the electrode on the semiconductor modulator, a terminal resistance is connected, in parallel, to the semiconductor modulation element.
U.S. Pat. No. 5,732,097 discloses a semiconductor modulation device configuration in which in order to cancel the electro-optical current generated by the semiconductor modulation element, a terminal resistance and a constant-current source are connected, in parallel, to the semiconductor modulation element. JP-A HEI 9-199778 discloses a semiconductor field absorption type optical modulation apparatus equipped with a semiconductor field absorption type optical modulator that uses a high-frequency electric signal input with an optical input from source to perform modulation. The disclosure offers higher modulation efficiency than a conventional modulator and consumes less power. The modulation apparatus includes an impedance conversion circuit that converts the impedance of the signal line that transmits the input high-frequency signal to a higher terminal-impedance for input to the optical modulator, and a terminal resistance having the above terminal impedance, that is connected to the input terminal of the modulator.
U.S. Pat. No. 5,995,270 discloses a configuration with a traveling-wave type semiconductor modulation device that allows interaction between the light wave and the high-frequency modulation signal over a long distance and a terminal resistance. However, reversing the direction of the light wave results in a major decrease in modulation efficiency, unlike in the case of the present invention and the above examples.
Compared to a configuration that does not use a terminal resistance, the above semiconductor optical modulators of the prior art achieve impedance matching and enable optical modulation with a small high-frequency signal. However, the terminal resistance consumes the major part of the high-frequency signal power. On the other hand, the invention of JP-A HEI 9-199778 achieves higher modulation efficiency by using impedance conversion to convert to a higher voltage. However, the large terminal resistance used to achieve impedance matching consumes high-frequency signal power. In order to use the modulator in the optical modulation section of a radio-on-fiber system, the consumption of high-frequency signal power has to be reduced.
The present invention was accomplished in view of the above, and has as its object to provide a resonance-type semiconductor optical modulator with an asymmetrical electrode structure having a high modulation effect, in which the power consumption of the microwave modulation signal is reduced through application the microwave power to the semiconductor optical modulation element through a resonator.
In the following description of the invention, an open-end stub means that the end of a microwave signal asymmetric coplanar waveguide that is not the signal-supply end is open-ended, and a short-ended stub means that the end is short-ended.
In accordance with a first aspect, the present invention attains the above object by providing a resonance-type semiconductor optical modulator with an asymmetrical electrode structure, comprising a semiconductor optical modulation element, an open-ended stub, a short-ended stub connected to the open-ended stub, a feeding line electromagnetically connected to the open-ended stub and short-ended stub, and a common electrode, in which the open-ended stub and short-ended stub are formed in mutual contact and the semiconductor optical modulation element is connected to the open-ended stub.
In a second aspect, the invention provides a resonance-type semiconductor optical modulator with an asymmetrical electrode structure, comprising a semiconductor optical modulation element, a first open-ended stub, a second open-ended stub connected to the first open-ended stub and having a different length than the first open-ended stub, a feeding line that is electromagnetically connected to the first open-ended stub and second open-ended stub, and a common electrode, in which the first open-ended stub and second open-ended stub are formed in mutual contact and the semiconductor optical modulation element is connected to an open-ended stub portion other than a portion connecting the feeding line to the open-ended stubs.
In a third aspect that includes the first aspect, a product of lengths of the open-ended stub and short-ended stub is a multiple integer of one-quarter a wavelength of a modulation signal, so that the modulation signal can be applied in a resonant state.
In a fourth aspect that includes any one of the first, second and third aspects, the semiconductor optical modulation element is connected to an open-ended stub end, so that effective modulation can be performed even when the stubs have a small length.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.